Re: Bi-directional One-to-one relationship
Re: Bi-directional One-to-one relationship
- Subject: Re: Bi-directional One-to-one relationship
- From: Michael Warner <email@hidden>
- Date: Sat, 16 Apr 2005 09:03:59 -0700
I finesse this issue in my apps by using a one-to-many relationship, then
constraining the the to-many direction with a tiny bit of code to only allow one entity to be related. Ugly as can be, but it works.
Also, have a look at
http://developer.apple.com/documentation/WebObjects/UsingEOModeler/Introduction/chapter_1_section_1.html
The chapter titled: Working With Relationships.
Mike W.
On Apr 16, 2005, at 8:53 AM, Drew Thoeni wrote:
Caveat emptor, but that's my understanding.
On Apr 16, 2005, at 11:50 AM, Des Oates wrote:
Hmm so what you're saying is that if the primary keys in each object have the same name, then you can do this? I never would have thought that that would work but I''ll definitely give it a try. Thanks for the tip.
Des
On 16 Apr 2005, at 16:30, Drew Thoeni wrote:
First let me preface by saying I'm an effective novice when it comes to WO, though I've been tinkering for a couple of years.
That being said, in my limited experience, generally you have a o-o relationship where EO1 and EO2 have the same primary key. I usually split a single table because of a BLOB or some such that I want to avoid referencing unless needed.
If I'm reading this right, your primary keys are PKData and PKState. I think the key should be the same in both tables, and skip having the separate foreign key.
So, DataEntity and StateEntity would both have a primary key of and PKState, and you'd control drag from PKState to PKData, selecting "to-one" in the inspector.
I'm pretty sure that would work and have done it in the past. I'd try it now but can't launch EOModeler due to 10.3.9 update :(
Drew
On Apr 16, 2005, at 10:14 AM, Des Oates wrote:
OK Sorry for not being clear, Here's an example.
I created 2 new EOs in a fresh EO model. DataEntity, and StateEntity.
In each a put a couple of Attributes and integer primary keys, PKData, and PKState respectively
To create a relationship I first copy PKState attribute, and paste it into DataEntity. I then cntr + drag from StateEntity.PKState to DataEntity.PKState.
By default that creates a One-to-many relationship. It inserts the relationship properties 'stateentity' and 'dataentitys' into DataEntity and StateEntity respectively.
I want to change this so I highlight StateEntity.dataentitys and open my inspector. I change the 'To Many' property to 'To One' in the relationship's inspector.
I also set its optionality to 'Mandatory' and its delete rule to no action.
On the other side of the relation ship (DataEntity.stateentity) I also set the optionality to Mandatory, set the delete rule to 'cascade' and the tick the 'owns destination'
If at this point I want to check the consistency of the model. I go to the Model Menu, and hit the 'Check Consistency' menu. This reports the following problem:
Relationship dataentitys in entity StateEntity is a to-one relationship, but its destination attribute(s) does not match the primary key of the destination entity.
Is this something I can live with? I don't believe so, since later on I get runtime errors when these objects are created relating to null primary keys, OR duplicate primary keys being created. I also get runtime errors if I create 2 independent and opposing unidirectional to-one relationships.
The only way I can get a one-to-one relationship to work is if I delete the relationship on the destination, which in this case would be 'StateEntity.dataentitys'. Then it works but of course its now unidirectional. I.e. I can't navigate to my DataEntity from my StateEntity. I want to be able to navigate to and from either object.
Sorry for being long winded, but that's about as concise as I could make it.
Des
On 16 Apr 2005, at 13:07, Drew Thoeni wrote:
If you are talking about setting up a o-o relationship, then yes EOmodeler will do this. Create the two tables, ctrl-drag from the primary key in one to the PK in the other and in the resulting window, click "to one".
I suspect you are, though, asking a more complex question. If you are, perhaps you can clarify with an example.
Drew
On Apr 16, 2005, at 8:01 AM, Des Oates wrote:
Does anyone know if EO Modeler supports a one-to one relationship where each related object is referenced as a property in the other? I've been trying to set up my EOModel like this for a while now, but no matter which way I try to do it EO Modeler complains.
A use case of this would be if you have a single data object, linked to its corresponding single state object. In O-O you would want to keep the logic handling these separate. (I.e. 2 different classes)
I ended up working around this so its not a showstopper for me but I'm just curious for future reference if anyone's actually managed to do it, and if so how.
thanks
Des
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Michael M. Warner, Ph.D.
Department of Educational Specialties/299
University of Nevada, Reno
Reno, NV 89557-0251
Phone: (775) 784-4383 x2039
Fax: (775) 784-4384
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